Muscles serve a number of functions, most of which are dependent upon their regular contraction, which is in turn dependent upon their strength and health. For example, in addition to the well known functions of supporting the skeleton and permitting movement, muscles serve to pad the force of bone protuberances against the skin, and they promote blood flow, particularly through deep blood vessels. In response to repeated contractions against a load, muscle fibers grow in cross-sectional area and develop more force, and in response to repeated contraction over a long period of time, the oxidative capacity and blood supply of the fibers is enhanced.
In normal individuals, muscles are activated to contract by electrical signals that are communicated from the brain and spinal cord by way of muscle nerves. Many medical diseases, physical disabilities and cosmetic disfigurements arise from abnormal or absent electrical signals to the muscles. Such abnormal or absent electrical signals may be pathological or may simply be due to prolonged immobility or confinement that restricts or prevents the voluntary movement of one or more muscles. Without normal, routine electrical stimulation, muscles atrophy, that is lose their normal size and strength. Also contributing to muscle atrophy may be a wide range of other pathophysiological mechanisms, including absence of sustaining hormones and other endogenous trophic substances.
Many situations exist in which voluntary muscle contraction cannot be used effectively to operate, condition or strengthen muscles. The most extreme loss of voluntary muscle function occurs when the brain or spinal cord is injured by trauma, the growth of tumors or cerebrovascular accidents. In patients suffering from these conditions, muscles become wholly or partially paralyzed because the electrical commands that are normally generated in the nervous system are no longer available to stimulate muscle contractions. Less extreme degrees of muscle weakness and atrophy can come about when some of the nerve fibers supplying a muscle are damaged by disease or injury, or when the muscle is immobilized or voluntarily rested, for example by casting or bedrest, in order to recover from an injury or surgical intervention involving a nearby body part, or other prolonged confinement or immobilization.
With respect to prolonged physical confinement or immobilization, the affect of muscle non-use and atrophy frequently leads to two disorders that are particularly difficult to avoid and expensive to treat, pressure ulcers of the skin and subcutaneous tissues and retardation of the normal circulation of blood through deep vessels. Continual, unrelieved pressure on localized regions of skin can result in the development of pressure ulcers of the skin and subcutaneous tissues, also known as bed sores or decubitus ulcers. Pressure ulcers are thought to occur when tissues underlying a site of pressure are deprived of oxygen and nutrients because blood flow is impeded, and when the area is subjected to frictional and shearing forces associated with continuous rubbing and movement. Pressure ulcers vary in size and degree of damage from small regions of redness to deep craters of tissue erosion passing through skin, connective tissues, muscle and even bone that can threaten the life of a patient by providing portals of entry for pathogenic organisms. They are often exacerbated in chronically paralyzed or bedridden patients because of atrophy of the unused muscles that normally provide a degree of padding between the skin and underlying bony protuberances. The treatment of pressure ulcers often requires prolonged, intensive medical care and occasionally extensive surgery, usually entailing further restrictions in the posture of the patient, which may further complicate medical and nursing care and cause other complications.
As mentioned above, prolonged immobilization or physical confinement of a body part often also results in retardation of circulation of blood through deep vessels, particularly the veins in an around muscles. For example, the failure to contract muscles in the limbs at regular intervals, as occurs normally when walking or standing, is known to cause stasis of blood in some veins. Venous stasis is a predisposing factor in the formation of clots in the veins. Such deep venous thrombosis further compromises blood flow to the immobilized body part and can be the source of dangerous emboli to the heart and lungs. Thrombosed veins may also become chronically infected, posing a danger of septicemia. Examples of particular populations of patients that are especially at risk for development of pressure ulcers and venous emboli include comatose and obtunded patients, patients who are confined by paralysis to bed or wheelchairs, bedridden patients who have medical or surgical conditions that limit their activity, and elderly patients with limited mobility. To reduce complications in these patients, it is necessary to reestablish movement of the vulnerable body parts; however, these patients are either incapable of voluntary movement or severely restricted in their ability to voluntarily move. Therefore, therapists often spend considerable time manipulating the passive limbs of these patients, but this is expensive and relatively ineffectual because it is the active contraction of muscle that tends to pump blood through the veins and to maintain the bulk of the muscle.
It has long been known that muscle contractions can be elicited involuntarily by stimulating muscles and their associated motor nerves by means of electrical currents generated from electronic devices called stimulators. This has given rise to various therapies that seek to prevent or reverse muscle atrophy and its associated disorders by the application of electrical stimulation to the muscles and their nerves via these stimulators. For example, the field of research known as functional neuromuscular stimulation (FNS) or functional electrical stimulation (FES) has begun, which seeds to design and implement devices capable of applying electrical currents in order to restore functional movement to paralyzed limbs. Similarly, therapies employing stimulators to regularly apply specific patterns of electrical stimulation to muscles in order to prevent or reverse atrophy are known.
Many of the earliest stimulators were bulky and relied upon the delivery of large current pulses through electrodes affixed to the skin, a procedure that requires careful positioning and fixation of the electrodes to the skin and frequently produces disagreeable cutaneous sensations and irritation of the skin. Additionally, such transcutaneous stimulation produces relatively poor control over specific muscles, particularly those that lie deep in the body. Thus, this procedure can be time-consuming, uncomfortable, and is generally useful only for muscles located immediately beneath the skin.
It is also possible to stimulate muscles more directly by passing electrodes through the skin into the muscles or by surgically implanting self-contained stimulators and their associated leads and electrodes in the body. These devices have many configurations, but most are large and have numerous leads that must be implanted and routed through the body to the desired muscles using complex surgical methods. Further, they are expensive to produce and the invasive procedures required for their implantation are impractical for most patients because they increase rather than decrease the required care and the danger of infection and other sources of morbidity in patients who are already seriously ill. Thus, such devices have been used primarily in patients with severe paralysis in order to demonstrate the feasibility of producing purposeful movements such as those required for locomotion, hand-grasp or respiration.
More recently a new technology has been described whereby electrical signals can be generated within specific tissues by means of a miniature implanted capsule, referred to as a "microstimulator", that receives power and control signals by inductive coupling of magnetic fields generated by an extracorporeal antenna rather than requiring any electrical leads. See, U.S. Pat. Nos. 5,193,539; 5,193,540; 5,324,316; and 5,405,367, each of which is incorporated in its entirety by reference herein. These microstimulators are particularly advantageous because they can be manufactured inexpensively and can be implanted non-surgically by injection. Additionally, each implanted microstimulator can be commanded, at will, to produce a well-localized electrical current pulse of a prescribed magnitude, duration and/or repetition rate sufficient to cause a smoothly graded contraction of the muscle in which the microstimulator is implanted. Further, operation of more than one microstimulator can be coordinated to provide simultaneous or successive stimulation of large numbers of muscles, even over long periods of time.
While originally designed to reanimate muscles so that they could carry out purposeful movements, such as locomotion, the low cost, simplicity, safety and ease of implantation of these microstimulators suggests that they may additionally be used to conduct a broader range of therapies in which increased muscle strength, increased muscle fatigue resistance and/or increased muscle physical bulk are desirable; such as therapies directed to those muscle disorders described above. For example, electrical stimulation of an immobilized muscle in a casted limb may be used to elicit isometric muscle contractions that would prevent the atrophy of the muscle for the duration of the casting period and facilitate the subsequent rehabilitative process after the cast is removed. Similarly, repeated activation of microstimulators injected into the shoulder muscles of patients suffering from stroke would enable the paretic muscles to retain or develop bulk and tone, thus helping to offset the tendency for such patients to develop subluxation at the shoulder joint. Use of microstimulators to condition perineal muscles as set forth in applicants' copending patent application, Ser. No. 08/007,521, filed Nov. 24, 1995, entitled "Method for Conditioning Pelvic Musculature Using an RF-Controlled Implanted Microstimulator", incorporated herein by reference, increases the bulk and strength of the musculature in order to maximize its ability to prevent urinary or fecal incontinence.
In addition to the therapeutic use of microstimulators to promote contraction of specific, isolated muscles in order to prevent or remedy the disorders caused or contributed to by inactive muscles, the administration of hormones, trophic factors and similar physiologically active compounds may also be useful. It is known that the extent to which a muscle will grow in response to any stimulation regime is affected by the hormonal and chemical environment around the muscle. Muscle fibers have receptors for many physiologically active compounds that circulate normally in the blood stream or are released from nerve endings. These trophic factors have significant effects on the nature, rate, and amount of growth and adaptation that can be expected of the muscle in response to stimulation, whether such is produced voluntarily or by electrical stimulation. Perhaps the best-known of these hormones are the androgenic steroids often used by athletes to increase muscle bulk and strength; but other hormones such as estrogens and growth hormones are also known to affect muscle properties. For example, the dramatic reductions in circulating estrogens and androgens that occur in women following menopause appear to account for decreases in the mass of muscles and bones, which can be slowed or even reversed by administering the deficient hormones systemically.
Thus, the beneficial strengthening effects of electrical stimulation can be maximized by providing the affected muscles with a supportive hormonal environment for growth. These compounds can be provided systemically by administering them orally or by injection. However, many such compounds are rapidly metabolized by the liver, so that high doses must be administered to achieve a desirable therapeutic effect. This can expose all tissues of the body, including the liver, to high and perhaps poorly controlled levels of the compound, resulting in undesirable side-effects that may outweigh the desired actions of the agent. In one aspect, the present invention recognizes that this problem could be circumvented by using a more selective method of drug delivery directed specifically to the electrically exercised muscles. Even if the introduced compound were ultimately to be cleared by absorption into the bloodstream, high concentrations would be produced only in the tissue around the target. A steep dilutional gradient would ensure that other regions of the body were exposed to much lower levels of the administered compound. By providing a more conducive chemical environment in the early stages of electrical therapy, it is expected that muscle atrophy could be reversed more rapidly and effectively. After muscle function has been reestablished, longer-term performance of the muscle could be more easily maintained at the desired level by electrical stimulation alone or in combination with low-dose systemic replacement therapy.
The microstimulators described and claimed herein are elongated devices with metallic electrodes at each end that deliver electrical current to the immediately surrounding biological tissues. The microelectronic circuitry and inductive coils that control the electrical current applied to the electrodes are protected from the body fluids by a hermetically sealed capsule. This capsule is typically made of a rigid dielectric material, such as glass or ceramic, that transmits magnetic fields but is impermeable to water vapor.
Encapsulation in glass is an effective and inexpensive way to ensure a hermetic seal between the electronic components and the biological tissues. Methods for forming similar hermetic seals within the confined dimensions of the overall device are well-known in the fabrication of industrial magnetic reed relays and diodes and have been described specifically for implantable microstimulators. See, e.g., U.S. Pat. Nos. 4,991,582; 5,312,439; and 5,405,367, each of which is incorporated in its entirety by reference herein. Such a hermetic barrier is important both to ensure good biocompatibility with the body and to protect the sensitive electronics from the body fluids that might destroy their function.
Unfortunately, however, glass and similarly brittle materials such as ceramic may crack or shatter as a result of externally applied forces or even residual stress in the crystalline structure of the material itself. If such an event occurs within the body or during a surgical procedure, it is desirable to retain or capture the sharp fragments of the capsule and any internal components so that they do not irritate or migrate into the surrounding tissues. In a testing or surgical environment in which devices are handled repeatedly, the hard, slippery surface of the glass capsule makes the device difficult to handle, and could increase the likelihood that the device will be dropped or pinched with a force sufficient to break the glass. Therefore, in one aspect, the present invention provides a well-chosen biocompatible coating for the glass which would decrease the lubricity of the device and ensure that glass pieces resulting from device fracture would be contained/captured in a protective sleeve.
The reaction of a living body to an intact foreign body such as an implanted microstimulator depends at least in part on the shape and texture of the surface of the foreign body, as described, e.g., by Woodward and Salthouse (1986). The surfaces left by the manufacturing processes used for the implanted microstimulator are constrained by the nature of the materials and processes required to achieve the desired electronic and mechanical characteristics of the device. Therefore, modification of the microstimulators' chemical nature and/or superficial physical contours to avoid, prevent and/or discourage an immunological response by the body, would be advantageous. Additionally, in selecting an appropriate coating material the opportunity arises for the introduction of various chemical compounds, such as trophic factors and/or hormones, as discussed above, into or onto the coating. Such compounds could then diffuse from the surface of the coating into the surrounding tissues for various therapeutic and diagnostic purposes, as previously mentioned.